This invention relates generally to air flow volume measuring apparatus in which the air flow is passed through a throat of known cross-sectional area and the volumetric measurement is determined as a direct function of air velocity, and more particularly to air flow volume measuring apparatus that are handheld and measure air volume with flow in either of the two directions through the known cross-sectional area throat.
Air flow volume measurements are performed in commercial and office buildings to balance heating, cooling and ventilating of the buildings. Balancing the air flow is desired to avoid portions of a building being hotter or colder, or more or less ventilated than other portions, and a building having well balanced air flow volumes is a more efficient user of heating, cooling and ventilating energy. Energy cost savings can be significant when the air flow volumes are well balanced.
These measurements are performed by measuring the volume of air per unit of time that passes through the air inlets and outlets throughout the building. From the measured data the heating, cooling and ventilating systems are adjusted to obtain desired flow rates to the different building portions, such as reception areas, hallways, individual offices and common areas. Long ago the measurements were performed by using a handheld air velocity measuring apparatus to determine the air velocity at an array of locations across any one inlet or outlet. The readings then were averaged and multiplied by a constant related to the type and size of the individual inlet or outlet to obtain a final reading. This process was time consuming and inaccurate.
Presently, the volume measurements are performed with an apparatus that directs the volume of air passing through an inlet or outlet through a throat of known area. The velocity of the air passing through the throat is used directly to indicate volume with a properly calibrated air velocity meter. The apparatus comprises a housing of sheet metal providing the throat of known area, and providing for mounting of additional elements thereon. A planar probe or manifold that is mounted in the throat transmits a representative sample of air entering the throat to a range selector and a velocity meter, and returns the air exhausted from the meter and selector to the volume of air exiting the throat. The velocity meter is a unidirectional device, air always passing through it in only one direction, and provides an indicator needle that is deflected at an angle proportional to the velocity of air passing through the meter. typically, this meter is the device disclosed and claimed in U.S. Pat. No. 3,463,003. The selector transmits the air to the velocity meter in the correct direction whether the air passes upward or downward through the throat and further extends the range of the velocity meter by throttling, in steps or ranges, the velocity of the air transmitted to the meter. A hood, made of woven material supported by rods, is attached to one end of the housing to aid in directing air flow through the throat. The hood further enables the meter to be at about eye level when the hood top is engaged around the periphery of a ceiling inlet or outlet.
In practice this prior apparatus is held by the operator with the hood top margin engaged around the inlet or outlet. The selector switch is rotated to the proper setting for the desired range and for air moving up or down through the apparatus, and the volume indicator needle is read. This prior apparatus is significantly more accurate in volumetric measurement than the prior method, but significant improvements therein are possible, mainly in the selector.
The selector of the prior apparatus comprises a laminated body of plastic plates having passages cut therein and a molded and cut disk rotatable in a central pocket of the body. The disk presents three radially extending throttling orifices of different cross-sectional diameter to throttle the air velocity passing therethrough and extend the range of the velocity meter. The disk is rotated to one side or the other of an OFF or null position depending on the direction of air flow through the throat and to align the desired throttling orifice with the body passages. The disk is held in a setting by a detenting ball and spring. Three calibration screws, one for each throttling orifice, are extendable axially of the disk into the throttling orifices to calibrate the apparatus.
In operation of the known selector, air flowing down through the throat forces air into the upper level of the manifold and into the selector, through the selected throttling orifice and across the throttling screw to the meter inlet tube. The return air from the meter outlet tube passes through a hollow in the selector disk and then exits from the lower level of the manifold to rejoin the air flowing down through the throat. Air flowing up through the throat first passes through the hollow in traveling to the meter and returns through the throttling orifice. This difference in flow paths results in a noticeable difference in measured air flow volumes depending on whether air is flowing down or up through the throat. The difference occurs because the air loses momentum and velocity as it winds through the manifold; the selector passages, orifice and hollow; and the velocity meter.
When the air passes through the throttling orifice on its way to the meter, there is a significantly different momentum and velocity loss than when the air passes through the throttling orifice returning from the meter, the orifice presenting the greatest constriction for the air flow through the manifold, selector and meter. Thus, calibrating the apparatus with air downflow results in noticeable error in measured volume with air upflow.
Additional error occurs due to a different air flow profile occurring through the throat depending upon the direction in which the air passes through the aperture. Air from the manifold indicates different velocities depending upon whether the throat air flow is up or down. Again, one throttling screw is inadequate to adjust for this error.
These two effects are additive and can cause a total error that swings 5%, i.e. calibrating the apparatus to zero error with throat upflow results in a 5% error in downflow air volume data. In a small building a 5% error in balancing heating, cooling and ventilating translates to a small total value in extra energy costs, but in a large building the extra energy costs can be enormous.
Additionally, the prior apparatus is limited in the resolution and repeatability from machine to machine of data obtained from the lower half, 250 cubic feet per minute (cfm) and less, of the meter's lowest volume range, 0-500 cfm. In practice, this is often the most useful portion of the meter ranges that the prior apparatus measures. It is used primarily in a large modern office building where there is a multitude of individual low volume air flow inlets and outlets rather than a single high volume air flow inlet and outlet.
Solving this problem requires more than simply providing an additional, twice as large cross-sectional area throttling orifice to provide the lower range. The lowest range throttling orifice already approaches the cross-sectional area of the passages through the selector, manifold and meter. Thus with a twice as large cross-sectional area throttling orifice, the passages through the manifold, selector and meter would act to throttle the air moving therethrough instead of the doubled area throttling orifice, which is undesirable.
The solution of the invention is not increasing the cross-sectional area of the throttling orifice but restricting the area of the throat with an additional element to cut in half the throat area and double the velocity of the air flowing through the throat. Caution must be exercised, however, to avoid disturbing the air flow pattern through the throat; a plate located in the throat with a central bore of half the area of the throat is unacceptable because it disturbs the normal air flow pattern through the throat and blocks some of the arrayed manifold entrances.
The invention provides a sheet of stock having an array of multiple, regularly spaced apart perforations, with the total of the free or open areas of the perforations being approximately equal to half of the area of the throat. Thus, the solidity or solid area of the sheet is also approximately half the area of the throat. Such a sheet or screen minimally affects the throat air flow pattern, doubles the throat air velocity, allows air access to the manifold entrances and is easily inserted and tension retained in the throat by the operator. The sheet easily is removable for changing to other volume ranges.
The use of inserts such as honeycombs, perforated plates and screens in direct flows is known to reduce or modify freestream turbulence in flowing gases. See Nagib, H.M.: Wray, J. L. and Tan-atichat, J., "Aeroacoustic Phenomena in Freestream Turbulence Manipulators," Progress in Astronautics and Aeronautics, Volume 37, 1975 and Loehrke, R.I.; Nagib, H.M., "Control of Free-Stream Turbulence by Means of Honeycombs: A Balance Between Suppression and Generation," Transactions of the ASME, Sept. 1976, Vol. 98, p.342 et seq., Journal of Fluids Engineering. The use of such inserts as area reducers and in particular as range extenders in air flow volume measuring apparatus is, however, unknown. None of this art suggests increasing the volume flow meter accuracy by increasing the accuracy of the sensing structure.